There are conventionally known laser microscope apparatuses that adopt a PMT (photomultiplier tube) as a photodetector (for example, see NPTL 1). In addition to a PMT, an MPPC (Multi-Pixel Photon Counter), a SiPM (Silicon Photomultiplier), and a PPD (pixelated Photon Detector), such as a multi-pixel avalanche photodiode, are known photodetectors.
When a PPD is used in an operation mode in which, upon detecting light, a signal with a certain intensity is output irrespective of the intensity of the light, that is, in Geiger mode, it is possible to detect weak light at the single photon level. When a PPD is used in Geiger mode, the number photons that can be detected per unit time is about one photon per pixel. Since the PPD has multiple pixels, even when the number of photons incident on the PPD is increased, it is possible to detect the corresponding number of photons.
The upper limit (saturation level) of the dynamic range of the PPD depends on the number of pixels on which light is incident, among the total pixels included in the PPD, and the saturation level becomes higher as the number of pixels on which light is incident becomes larger. Specifically, the saturation level of the PPD depends on the beam diameter of light incident on the PPD, and the saturation level of the PPD becomes higher as the beam diameter becomes larger. The beam diameter varies depending on an observation condition, such as the exit NA (numerical aperture) of an objective lens. Therefore, for example, when the user changes the objective lens from one with a large exit NA to one with a small exit NA, the saturation level of the PPD is lowered.